Add baselining mechanism to the build comparison tool

[mmitche]

Accepts an optiona; json file with a list of baseline entries. Baseline a few well known items.

[Copilot]

Pull Request Overview

This PR introduces a baselining mechanism to the build comparison tool. Key changes include:

• Adding new types (BaselineEntry, Baseline, and associated JSON converter) to support baseline configuration.
• Updating the CLI options and Program.cs to load and apply baselines during asset evaluation.
• Updating the pipeline YAML to pass the baseline file path.

Reviewed Changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/SourceBuild/content/eng/tools/BuildComparer/AssetType.cs	New enum for asset types.
src/SourceBuild/content/eng/tools/BuildComparer/Issue.cs	New class representing asset issues with baseline support.
src/SourceBuild/content/eng/tools/BuildComparer/AssetMapping.cs	New class for mapping asset details and tracking issues.
src/SourceBuild/content/eng/tools/BuildComparer/ComparisonReport.cs	New report class summarizing issues, errors, and baselined items.
src/SourceBuild/content/eng/tools/BuildComparer/BaselineEntry.cs	New baseline classes and a regex JSON converter for baseline configuration.
src/SourceBuild/content/eng/tools/BuildComparer/IssueType.cs	New enum for issue types.
src/SourceBuild/content/eng/tools/BuildComparer/Program.cs	Updated to add CLI options, load baseline file, apply baselining, and update reporting.
eng/pipelines/templates/steps/vmr-validate-asset-baseline.yml	Pipeline template updated to include the baseline argument.

Comments suppressed due to low confidence (1)

src/SourceBuild/content/eng/tools/BuildComparer/Program.cs:50

• The baseline CLI option is marked as required, but the PR description indicates that the baseline file should be optional. Consider setting Required = false.

var baselineArgument = new CliOption<string>("-baseline") { Description = "Path to the baseline build manifest.", Required = true };